feat(runtime): add integration hardening and diagnostics

docs/PROTOCOL.md

@@ -37,6 +37,9 @@ is only echoed for correlation; it grants no authority.
   unpaused match may submit movement intent.
 - `drop_key`: `{}`. Only a player in an active, unpaused match may submit the
   key-drop intent.
+- `restart`: `{}`. Only connected Player 1 may reset a finished match, and both
+  players must be connected. Sessions are preserved while a new authoritative
+  maze and game state are created.
 - `heartbeat`: `{}` after a session has been joined or restored.
 
 ## Server messages
@@ -57,6 +60,8 @@ is only echoed for correlation; it grants no authority.
 - `resume`: reports that both players are attached and authoritative gameplay
   has resumed.
 - `match_result`: reports the terminal winner or draw.
+- `reset`: reports the authoritative return from a finished match to the
+  waiting lobby.
 - `ack`: successful heartbeat correlation.
 - `error`: a stable English `code` and Russian player-facing `message`.
 
@@ -67,3 +72,13 @@ so a refresh can restore it. A player disconnect pauses the game for at most 60
 seconds; restoration reattaches the same state, while expiry produces a draw.
 A disconnected observer token may be restored until its bounded slot is reused
 by a new observer, and observer disconnects never pause the match.
+
+## Runtime diagnostics
+
+`GET /diagnostics` returns bounded operational counters and internal-memory
+samples without exposing session tokens, Wi-Fi credentials, or hidden maze
+state. Counters cover connection and queue high-water marks, sent/dropped/
+failed messages, malformed and rate-limited input, accepted game commands,
+match cycles, maximum WebSocket handler/send duration, and current/minimum
+internal free heap and largest block. The response also publishes the fixed
+limits used by the Milestone 010 physical stress client.